Control method and optical data transmission path for compensating changes in SRS-induced power exchange

ABSTRACT

Control method and optical data transmission path having a device for determining the tilting of the spectrum, and having a quick control and slow control for compensating the tilting of the spectrum.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a control method forcompensating changes in the SRS-induced power exchange when connectingchannels into, and disconnecting them from, a continuous optical datatransmission path of a WDM system by influencing the tilting of thespectrum. Furthermore, the present invention relates to an optical datatransmission path having a WDM system with a multiplicity of datatransmission channels of different frequencies with at least onemultiplexer, arranged at the beginning, for combining the datatransmission channels, one demultiplexer arranged at the end, forseparating the data transmission channels, and at least one pathsection, arranged therebetween, having capabilities for determining andcompensating the spectral tilting of transmitted data signals.

[0002] It is known that stimulated Raman scattering (SRS) leads to apower exchange between the individual wavelength channels of awavelength multiplex system (WDM system). Channels with relatively largewavelengths experience an increase in their medium power here, while theaverage power of channels with relatively small wavelengths decreases.This effect of SRS can be counteracted in the steady state of a datatransmission path with WDM system by “tilting” the gain spectrum of anerbium-doped fiber amplifier (EDFA), for example using mechanicallycontrollable filters, as is known from the U.S. Pat. No. 5,847,862.However, a problem here is the time when channels are connected ordisconnected during operation.

[0003] Another is the failure of individual channels. Both thecontrollable filters and the erbium-doped fiber amplifiers are too slowin their reaction in order to be able to react quickly to the rapidintensity changes resulting from the connection and disconnection ofindividual channels or of a number of channels. Thus, during thetransmission of data, time periods in which the noise/signal ratio istoo low and the bit error rate of at least individual channels risesoccur repeatedly. This leads then to a reduced data rate in these datatransmission paths.

[0004] An object of the present invention is, therefore, to develop amethod and a device which permit quicker compensation of the tilting ofthe spectrum during the connection or disconnection of channels, or whenchannels fail, in a data transmission path with WDM system.

SUMMARY OF THE INVENTION

[0005] The inventors of the present invention have recognized that it ispossible to compensate the short-term and small intensity fluctuationsin a data transmission path which lead to a change in the tilting of thetransmitted spectrum of the data signals in the data transmission pathby virtue of the fact that one or more full lasers are used tocompensate these intensity fluctuations immediately, and “compensation”for this change by the full laser then takes place slowly in such a waythat the existing slow compensation mechanisms of the tilting can becompensated. It is not necessary here for the original spectrum of thedata signals to be retained, but rather it is sufficient if the overallintensity remains within a specific bandwidth of approximately 100 nm,and the full laser is maintained in this region, which can be locateddifferently depending on the property of the fiber used. For thiswavelength dependence, reference is made to M. Zirngibl, “Analyticalmodel of Raman gain effects in massive wavelength division multiplexedtransmission systems”, Electron. Lett., Vol. 34, pp. 789-790, 1998.

[0006] In accordance with these inventive ideas described above, theinventors of the present invention propose to improve the known controlmethod for compensating changes in the SRS-induced power exchange whenconnecting channels into, and disconnecting them from a continuousoptical data transmission path of a WDM system by influencing thetilting of the spectrum, to the effect that the tilting is brought aboutvia at least two systems which operate at different speeds, with atleast one quicker system measuring a change in the overall power in theoptical data transmission path and compensating the tilting by changingthe power of an injected full light source. Full light source within theterms of the present invention is to be understood as anyenergy-supplying light source which amplifies an optical signal. Inparticular, this may be a full laser or a broadband light source, forexample a white light source whose spectrum is, if appropriate,constricted by a filter.

[0007] In one particularly advantageous embodiment of the method of thepresent invention, a time delay is generated in the signal in theoptical path between measurement of the overall power and injection ofthe full light source so that the reaction time between the measurementof the overall intensity and the response of the full light source iscompensated.

[0008] According to the present invention, this control method can beapplied together with a slow method for influencing the tilting of thespectrum via controllable filters or power-controlled EDFAs.

[0009] In addition, it is advantageous if the quickly operating systemfirstly compensates changes quickly for influencing the tilting and thenreturns slowly to the original state, the more slowly operating systemperforming this compensation.

[0010] The full laser can be injected at the start of the opticaltransmission path, or else at the end of the optical transmission pathand injected counter to the direction of transmission.

[0011] It is particularly advantageous to use at least two full lightsources or full lasers instead of one full light source or full laser.This makes it possible to compensate not only the tilting but also thechange in the Raman gain averaged over all the signals.

[0012] If the entire bandwidth used exceeds 100 nm, it is necessary toensure that the power remains constant in subbands which each have abandwidth of less than 100 nm. To do this, a correspondingly largernumber of full lasers must be used and the overall power per subbandmeasured, it being possible to use monitor diodes which measure thepower in one subband each. The subbands here must in total cover theentire wavelength range used. It is advantageous if the subbandsoverlap.

[0013] Of course, it is also possible if, for example, the datatransmission path is composed of a number of path sections which are nottransparent with respect to one another, to use the method describedabove for each individual path section.

[0014] In accordance with the method described above, the inventors ofthe present invention also propose to supplement an optical datatransmission path having a WDM system with a multiplicity of datatransmission channels of different frequencies with at least onemultiplexer, arranged at the beginning, for combining the datatransmission channels, one demultiplexer arranged at the end, forseparating the data transmission channels, and at least one pathsection, arranged therebetween, having capabilities for determining andcompensating the spectral tilting of transmitted data signals in such away that provisions are made which are assigned to at least one pathsection for indirectly or directly measuring the overall intensity ofthe transmitted light signal, one or more controlled full light sourceor sources for injecting light power into at least one path section, andfurther provisions are made for controlling the power of the full laserin order to compensate power fluctuations of the overall intensity ofthe data signal.

[0015] Here, one advantageous embodiment includes arranging theprovisions for indirectly or directly measuring the overall intensity ofthe transmitted light signal and the controlled full laser for injectinglight power at the beginning of a path section, preferably at thebeginning of the entire data transmission path.

[0016] Furthermore, it is possible for a delay element to be arrangedbetween the provisions for measuring the overall intensity and the fulllight source or sources, which delay element may be, for example, adispersion-compensating fiber (DCF) which is used in the datatransmission path and in the booster.

[0017] The present invention also includes equipping an optical datatransmission path with a control device which is suitable for carryingout the control method described above. This also may, in particular,include a microprocessor with suitable data memories and programmemories, it being possible to provide programming for carrying out themethod according to the present invention. However, a correspondinganalog control which is more costly also lies within the scope of thepresent invention.

[0018] In one further advantageous embodiment of the optical datatransmission path according to the present invention, it is possible toprovide for the at least one frequency of the full light source or ofthe full laser to be located within the transmitted wavelength band ofthe transmitted data signals. A full laser can preferably have a singlefrequency.

[0019] As already mentioned in the control method, the provisions forcompensating the spectral tilting of transmitted data signals can havecontrollable frequency-dependent filters or power-controlled EDFA.

[0020] Furthermore, one particularly advantageous embodiment of theoptical data transmission path can be provided in which the provisionsfor determining the spectral tilting of transmitted data signals in thepath sections have at least one filter or amplifier withfrequency-dependent transmission characteristic or gain characteristicand downstream overall intensity meters, including an evaluation unitfor determining the tilting. For this particular embodiment of themeasuring device and method of measuring the tilting of the spectrum,reference is made to the simultaneously submitted patent application bythe applicant with the title “Verfahren und Vorrichtung zur Bestimmungund Kompensation der Verkippung des Spektrums in einer Lichtleitfasereiner Datenubertragungsstrecke” [Method and device for determining andcompensating the tilting of the spectrum in an optical waveguide of adata transmission path], and its disclosed contents with respect to themeasuring method of the tilting are incorporated fully.

[0021] Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following detaileddescription of the invention and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 shows a schematic view of the present invention withreference to an optical data transmission path.

[0023]FIG. 2 shows a view of the control concept of the presentinvention.

[0024]FIG. 3 shows the variation over time of the control variables whenchannels are connected.

[0025]FIG. 4 shows an alternative embodiment of a data transmission pathaccording to the present invention with an controllable filter.

DETAILED DESCRIPTION OF THE INVENTION

[0026]FIG. 1 shows a schematic view of an embodiment of an optical datatransmission path according to the teachings of the present invention.Here, a multiplicity of data transmission channels 1.1 to 1.4 arecombined via a multiplexer 2. A constant extracted part of the overallintensity of the transmitted light power is then measured in a monitor 3via a coupler 4. In accordance with the result of the intensitymeasurement, a full laser 6, which is operated at a medium power levelif no quick compensation measures are necessary, that is to say in thesteady state, is controlled so as to perform initial compensation of thepower fluctuations on the basis of the power fluctuations measured. Thepower of the full laser 6 is injected downstream of a time delay element5 in the direction of transmission via a wavelength-selective coupler 7.This is then followed by a generally known path section 8 of a datatransmission path with a tilting control via a power-controlled EDFA 8.1and the transmission fiber 8.2 which spans the actual subsequentdistances. A demultiplexer 9 finally separates the data transmissionchannels 10.1 to 10.4 which are converted into electrical signals withthe receivers 11.1 to 11.4.

[0027] The control method for quickly compensating the changes in theSRS tilting proceeds as follows. It is assumed that the system is in thesteady state and the full laser 6 is outputting a medium power P₀. Atthe output of the multiplexer 2 the overall power is measured in themonitor 3. If the measuring device detects a change in the overall powerover time, the power of the full laser 6 is correspondingly increased ordecreased so that the power at the input of the transmission path 8remains constant. Because the control of the full laser 6 requires acertain amount of time, the signals are delayed by this time period by adelay element 5 after the detection of their overall power. For thedelay which is necessary, it is possible, for example, to use in thetransmission over standard fibers the dispersion-compensating fiberwhich is present in any case in the booster. Of course, the overallpower also can be determined by measuring the output power of all thetransmitters 12.1 to 12.4 upstream of the multiplexer 2 and adding them.In addition, the power which is output by the full laser also can beinserted at the end of a booster which is not explicitly illustratedhere.

[0028] The wavelength of the full laser 6 is best selected here in sucha way that it lies within the transmitted wavelength band. Here, use ismade of the particular property of SRS that the tilting depends only onthe overall power occurring within a wavelength range of approximately100 nm, irrespective of how this overall power is distributed among theindividual channels. For this reason, a full laser with a singlewavelength is sufficient for the control purposes.

[0029] A way of integrating the described control into the controlconcept, known per se, of tilting compensation in the transmission pathis illustrated in FIG. 2.

[0030] The slow control outputs to the N EDFA 8.1.1 to 8.1.N of thetransmission path 8 control signals 15.1 to 15.N which predefine itstilting. At the same time, a setpoint signal 14.1 is generated for thequick control 14. If the signal 14.2 of the overall power measured viathe monitor 3 then changes, this is first compensated by the quickcontrol 1 4 by changing the power of the full laser via the actuationsignal 14.3. The deviation from the setpoint value is, however, alsoreported to the slow controller 1 3 via the signal 14.4. The slowcontroller 1 3 then reacts by outputting, in small steps, commands tothe EDFA 18.1.1 to 18.1.N to adapt the tilting, and at the same timeadapting the setpoint value for the control via the line 14.5. Thisadaptation mechanism is continued until the output signal of thecomparator 19 disappears. As a result, a new steady state is establishedin which the full laser outputs the medium power P₀ again.

[0031] The variation in the control variables over time when connectingchannels is illustrated by way of example in FIG. 3, the left-hand graypart representing the initial steady state and the right-hand gray timesegment representing the steady state after the control phase has ended.

[0032]FIG. 3 shows different measurement and control values of thecontrol according to the present invention as a functional profilecoordinated chronologically over the same time axis. At the beginning,from to to t1, and at the end, to the right of t2, of the time axis, theold and new steady states are shown with gray backgrounds. At the top,the variation over time of the overall power 20 measured at the monitor3 in FIG. 1 is represented, the overall power 20 rising at the end ofthe first gray area suddenly owing to the connection of the channels atthe time t1. Below that, the value 21 of the signal 14.3 for actuatingthe full laser 6 is shown, and below that the profile of the value 22 ofthe setpoint value 14.1 of the quick control 14, and finally below thatthe magnitude of the value 23 of the control signal for tilting the EDFA15.1 to 15.N from FIG. 2 is plotted.

[0033] The gain of the EDFA 8.1.1 to 8.1.N is also affected by changesin the input power. However, in contrast to the SRS, the gain of an EDFAreacts relatively slowly to changes in the input power so that it issufficient to adapt the pumping power injected into the doped fibers.

[0034] The integration of the quick control 14 into the slow control 13serves to limit the value range of the output power of the full laser.In a WDM system with, for example, 80 channels in a wavelength band, thefull laser would have to be capable of outputting an output power of upto 80 times the power of a channel. This then results in massivecrosstalk problems at the demultiplexer 9, even if the full laser 6 haslarger wavelength spacing with respect to the signal lasers 1 2.1 to12.4 than they have with respect to one another. This is the case, forexample, if the full laser 6 is positioned in a band gap in which thereare no signals for the purpose of subband dispersion compensation. Onthe other hand, if there is restriction to dealing only with thesimultaneous failure of a small number of lasers, for example 16, thefull laser 6 only has to be capable of outputting 16 times the power ofa channel, and the crosstalk problems can be made negligible.

[0035] If the steady state is restored after compensation has beencarried out, a small number of lasers may be allowed to fail again, orchannels may be connected or disconnected. This embodiment of thecontrol enables the crosstalk problems to be overcome relatively easily.

[0036] In the described form of the method of the present invention, itis necessary that the transmission path be transparent at the wavelengthof the full laser. If this is not the case, further full lasers must beprovided in each case downstream of the separation points which theoptical data signal cannot pass and at which it is regenerated.

[0037] An alternative embodiment of a data transmission path accordingto the present invention is illustrated in FIG. 4. In this case, thequick control is integrated into each of the boosters which aregenerally composed of a number of stages. In the present case it isassumed that there is a dispersion-compensating fiber (DCF) between thetwo amplifier stages illustrated. A change in the overall power isreacted to in that the power of the full laser which is injectedcontradirectionally into the DCF is appropriately adapted.

[0038]FIG. 4 shows the basic design of an optical amplifier, which istypically composed of two amplifier stages 18 between which there is afiber for dispersion compensation and the device for compensating theSRS. At the beginning, a constant part of the transmitted light power isextracted via a coupler 4, measured in a monitor 3, and the result issignaled to the controls 13/14. The controls 13/14 control, on the onehand, the slowly reacting influencing of the tilting via a controllablefilter (gain tilt filter) 16 and, on the other hand, the full laser 6.The power of the full laser 6 is injected downstream of adispersion-compensating fiber 17, counter to the direction of datatransmission via a wavelength-selective coupler 7.

[0039] In summary, the method according to the present invention and thedata transmission path described permit quicker compensation of thetilting of the spectrum when connecting and disconnecting channels orwhen channels fail in a data transmission path with a WDM system than inthe prior art.

[0040] Although the present invention has been described with referenceto specific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

1. A control method for compensating changes in an SRS-Induced PowerExchange when connecting channels into, and disconnecting channels from,a continuous optical data transmission path of a WDM system, the methodcomprising the steps of: providing at least two systems which operate atdifferent speeds to influence tilting of a spectrum of data signals inthe optical data transmission path; measuring a change in overall powerin the optical data transmission path via at least one quicker system ofthe at least two systems; and compensating the tilting by changing apower of at least one injected full light source via the at least onequicker system.
 2. A control method for compensating changes in anSRS-Induced Power Exchange as claimed in claim 1, the method furthercomprising the step of: incorporating a time delay in the signal in theoptical data transmission path between measurement of the overall powerand injection of the at least one full light source.
 3. A control methodfor compensating changes in an SRS-Induced Power Exchange as claimed inclaim 1, the method further comprising the step of: providing acontrollable filter, wherein the influencing of the tilting of thespectrum is additionally performed by the controllable filter.
 4. Acontrol method for compensating changes in an SRS-induced Power Exchangeas claimed in claim 1, further comprising: power-controlled EDFA,wherein the influencing of the tilting of the spectrum is at leastadditionally performed by the power-controlled EDFA.
 5. A control methodfor compensating changes in an SRS-Induced Power Exchange as claimed inclaim 1, wherein the at least one quicker system performs the step ofcompensating the tilting quickly, and a slower system of the at leasttwo systems then returns the compensating of the tilting slowly in adirection of an original state.
 6. A control method for compensatingchanges in an SRS-Induced Power Exchange as claimed in claim 1, whereinthe at least one injected full light source is injected at a start ofthe optical data transmission path.
 7. A control method for compensatingchanges in an SRS-Induced Power Exchange as claimed in claim 1, whereinthe at least one injected full light source is injected at an end of theoptical data transmission path and counter to a direction oftransmission.
 8. A optical data transmission path having a WDM systemwith a plurality of data transmission channels of different frequencies,comprising: at least one multiplexer, arranged at a beginning of theoptical data transmission path, for combining the data transmissionchannels; a demultiplexer, arranged at an end of the optical datatransmission path, for separating the data transmission channels; and atleast one path section arranged between the at least one multiplexer andthe demultiplexer for determining and compensating spectral tilting oftransmitted data signals, the at least one path section including a partfor measuring an overall intensity of the transmitted data signals, atleast one controlled full light source for injecting light power intothe at least one path section, and a part for controlling power of thefull light source to compensate power fluctuations of the overallintensity of the transmitted data signals.
 9. An optical datatransmission path as claimed in claim 8, wherein both the part formeasuring the overall intensity of the transmitted data signals and theat least one controlled full light source are arranged at a beginning ofthe at least one path section.
 10. An optical data transmission path asclaimed in claim 8, further comprising: a delay element provided betweenthe part for measuring the overall intensity of the transmitted datasignals and the at least one controlled full light source.
 11. Anoptical data transmission path as claimed in claim 1 0, wherein thedelay element is selected from the group consisting of adispersion-compensating fiber, a fiber with low dispersion, and a fiberdoped with a rare earth element.
 12. An optical data transmission pathas claimed in claim 8, wherein all of the parts of the at least one pathsection are provided as a control element which can be influencedquickly.
 13. An optical data transmission path as claimed in claim 8,wherein a frequency of the at least one controlled full light sourcelies within a transmitted waive length band of the transmitted datasignals, and the at least one controlled full light source has a signalfrequency.
 14. An optical data transmission path as claimed in claim 8,wherein the at least one path section includes frequency-dependentfilters which can be controlled in the at least one path section forcompensating the tilting.
 15. An optical data transmission path asclaimed in claim 8, wherein the at least one path section includespower-controlled EDFA for compensating the tilting.
 16. An optical datatransmission path as claimed in claim 8, wherein the at least one pathsection includes at least one element, which is one of a filter and anamplifier, with a respective frequency-dependent transmissioncharacteristic and a game characteristic, as well as downstream overallintensity meters, including an evaluation unit for determining thetilting.